Seniors and children under age five are the most vulnerable, with hospitalization rates about double the national average. A flu variant that’s more severe for the elderly is also dominant right now, which means the U.S. could be in for a tough season. More than 60% of flu samples tested by public health labs were positive for the influenza A(H3N2) strain, according to CDC.

“It is a well described phenomenon. H3N2 has a more severe impact on older persons so more hospitalization, ICU admissions and deaths,” said Dr. William Schaffner, an infectious disease expert at Vanderbilt University.

Flu vaccines typically aren’t as effective against H3N2, though there’s hope that this season might prove different. The majority of flu viruses tested are similar to the strains included in this year’s vaccine, according to the CDC.

Vaccine efficacy data hasn’t been published yet, but the shots normally perform better when they are matched well to the circulating variants. Flu vaccine efficacy has ranged widely from 19% to 60% in past seasons depending on how well the shots were matched to the strains circulating.

“From what we can see, it looks like the vaccines are pretty darn good matches to what’s circulating,” Hensley said. “If there’s ever a time to get vaccinated, this is the year to do it,” he said.

Flu activity was highest in the Southeast in past weeks, but most of the country is now seeing high levels of illness, according to CDC.

Flu activity is moderate or low in Alaska, Arizona, Delaware, Hawaii, Iowa, Maine, Massachusetts, Michigan, Montana, New Hampshire, Pennsylvania, North Dakota, Rhode Island, Vermont, Wisconsin, South Dakota and Wyoming.